1. Field of the Invention
The invention relates to the field of multimedia communication systems and, more specifically, to the minimization of lip sync errors induced by variable delay transport networks.
2. Background of the Invention
The problem of “lip synchronization” or lip sync is well known. Briefly, temporal errors in the presentation of audio and video streams by a presentation device may result in a condition whereby audio information is presented before (leading) or after (lagging) corresponding video information, resulting in, for example, poor synchronization between the audio representation of a speaker's voice and the video representation of the speaker's lips.
Prior art techniques to solve the so-called lip sync problem are relatively complex, and sometimes cause degradation in the audio and/or video information. For example, it is known to drop video frames such that a temporal advance of video imagery is induced to thereby correct for a leading audio signal.
Lip sync errors may be caused by many sources. Of particular concern is the use of variable delay networks such as the Internet and other packet switching networks. In such networks, audio and video information is transported as separate and independent streams. During transport processing prior to the introduction of these streams to the variable delay network, a transport layer header containing a timestamp as well as other metadata (e.g., encoder sampling rate, packet order and the like) is added to some or all of the transport packets. The timestamps for the audio and video information are typically derived from a common source, such as a real-time clock. Unfortunately, as the audio and video packets traverse the variable delay network, temporal anomalies are imparted, packets are dropped, order of the packets is not preserved and packet delay time is varied due to network conditions. The net result is lip sync error within received audio and video streams that are passed through the variable delay network.